If a table constraint, list of columns which the constraint
constrains.

confkey

int2[]

pg_attribute.attnum

If a foreign key, list of the referenced columns.

conbin

text

If a check constraint, an internal representation of the
expression.

consrc

text

If a check constraint, a human-readable representation of the
expression. This is not updated when referenced objects change; for example, it won't
track renaming of columns. Rather than relying on this field, it is best to use
pg_get_constraintdef() to extract the definition of a check
constraint.